The present invention relates to a cathodoluminescent display means using guided electrons and its control process. The invention more particularly applies to the production of displays permitting the display of fixed or moving pictures.
Known cathodoluminescent screens or displays are the cathode ray tube, the vacuum fluorescent display (V.F.D.) and the microdot fluorescent screen or display. The main known characteristics of these types of cathodoluminescent screen or displays will now be described.
The cathode ray tube has a vacuum cell with a thick glass plate (several centimetres) of limited curvature, a thinner glass envelope (approximately 1 cm) and which is approximately conical and tightly sealed to the thick glass plate and an electron gun located in the narrow terminal part of the conical envelope. Coils outside the conical envelope ensure the electromagnetic deflection of the electron beam from the gun. The electrons strike a cathodoluminescent layer deposited on the inner face of the thick glass plate, which is located in the vacuum enclosure and raised to a potential of several dozen kilovolts. The cathodoluminescent material is, for example, zinc sulphide.
The minimum depth of the cathode ray tube is on the one hand given by the aperture angle of the conical envelope directly linked with the maximum electromagnetic deflection angle of the electron beam and on the other hand by the minimum length of the actual electron bean. The ratio of the diagonal of the display to the depth remains, in the case of the conventional cathode tube, less than two. Therefore, the cathode tube does not constitute a fiat screen, even when the glass plate with the face forwards is approximately planar.
Moreover, with the useful size of the screen there is also an increase in the respective thicknesses of the plate and the envelope, both made of glass and therefore, a corresponding increase in the weight of the screen or delay. A cathode ray tube whereof the screen diagonal is 1 meter, at present weighs about 130 kg.
The vacuum fluorescence display has a vacuum cell incorporating two planar glass plates sealed by a tight cord or bead. The first slab optionally carries on its inner face located in the vacuum enclosure, a single electrode constituting an earth plane. Between the two plates within the vacuum enclosure there are in two successive planes two metal conductor levels.
At the first level, close to the first plate, are located taut metal wires heated by the Joule effect. They constitute cathodic filaments emitting electrons by the thermoionic effect. At the second level, close the second plate, are provided perforated, taut, metal strips. They constitute gate electrodes extracting and accelerating the electrons when they are selected, i.e. raised to a sufficiently high potential.
The second plate carries on its inner face located within the vacuum enclosure an array of transparent electrodes covered with an electroluminescent and slightly conductive material. They constitute anodes collecting the electrons extracted in their vicinity by a selected gate when their potential is sufficiently high and in particular higher than the respective potentials of the gates.
By striking the cathodoluminescent material the electrons bring about a light emission. The cathodoluminescent material is, for example, zinc sulphide. The filaments are, for example, of tungsten, the gates of aluminium and the anodes of indium oxide. The vacuum luminescent screen or display has a limited electron emission yield or efficiency outside the filaments. The filaments cannot be raised to high temperatures favourable for high efficiency levels, because they would be luminous and therefore visible.
The vacuum fluorescent display can only offer an image of limited size. The intermediate metallic levels Of taut gates and filaments require mechanical supports. They would be visible when located inside the useful zone of the display. Located at the periphery of the display they cannot prevent the filaments and gates from bending under the effect of their own weight and thermal expansion and the plates under the effect of the atmospheric pressure. The greater the size of the display, the greater the bending effects. The surface of the vacuum fluorescent displays is at present limited to a few square decimeters.
The microdot fluorescent display is known and described in the report of the international congress "Japan Display 86", p 512. The microdot fluorescent display incorporates a vacuum cell having a first glass plate on which are deposited cathode conductors supporting metal microdots. The cathode conductors are separated from a gate conductor deposited on the same plate by an electrically insulating layer. The gate conductor and the insulating layer are perforated in front of each microdot. The thus formed openings permit the passage of the microdots. A fluorescent material layer faces the gates and is deposited on the anode conductors, which rest on the second glass plate.
The two glass plates are tightly sealed so as to form a vacuum enclosure. Microspacers, positioned between the two glass plates, bear on the one hand on the metal of the gates of the first plate and on the other on the cathodoluminescent material of the second plate, making it possible to maintain a uniform distance between the two plates no matter what the surface of the cell and even when the glass of the plates is approximately 1 mm.
The cathodoluminescent material is, for example, zinc sulphate, the material of the cathodes and the anodes being of tin-doped indium oxide, while the material of the dots is molybdenum and that of the gates aluminium. The microspacers are, for example, calibrated glass balls with a diameter of approximately 200 microns.
The microdot fluorescent display has significant brightness inhomogeneities. The production of microdots with a height of approximately 1 micron and a base diameter of approximately 1 micron, uses microelectronic procedures. The present produced microdot fluorescent displays have a size less than 10 cm diagonal.